Add shadcn/ui design system, theme, and base components

Install shadcn/ui with base-nova style, tailwind-merge, tw-animate-css,
and font packages. Add oklch-based light/dark theme in index.css and
17 base UI components (button, card, dialog, table, tabs, etc.) plus
shared lib utilities (format, colors, cn) and useSettings hook.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/ui/textarea.tsx

@@ -0,0 +1,18 @@
+import * as React from "react"
+
+import { cn } from "@/lib/utils"
+
+function Textarea({ className, ...props }: React.ComponentProps<"textarea">) {
+  return (
+    <textarea
+      data-slot="textarea"
+      className={cn(
+        "flex field-sizing-content min-h-16 w-full rounded-lg border border-input bg-transparent px-2.5 py-2 text-base transition-colors outline-none placeholder:text-muted-foreground focus-visible:border-ring focus-visible:ring-3 focus-visible:ring-ring/50 disabled:cursor-not-allowed disabled:bg-input/50 disabled:opacity-50 aria-invalid:border-destructive aria-invalid:ring-3 aria-invalid:ring-destructive/20 md:text-sm dark:bg-input/30 dark:disabled:bg-input/80 dark:aria-invalid:border-destructive/50 dark:aria-invalid:ring-destructive/40",
+        className
+      )}
+      {...props}
+    />
+  )
+}
+
+export { Textarea }